1. Statement of the Technical Field
The invention concerns frequency selective surfaces (FSSs). More particularly, the invention concerns FSS based devices and methods of making the same.
2. Background
FSSs are surface constructions generally comprising a periodic array of electrically conductive elements. As known in the art, in order for its structure to affect electromagnetic waves (EMs), the FSS must have structural features at least as small, and generally significantly smaller, as compared to the wavelength of the electromagnetic radiation it interacts with.
FSSs are typically used in a variety of antenna applications. Such antenna applications include, but are not limited to, radome applications, Dichroic sub-reflector applications, reflect array lens applications, spatial microwave applications, optical filter applications, radio frequency identification (RFID) tag applications, collision avoidance applications, waveguide applications, and low probability of intercept system applications.
A schematic illustration of a conventional multi-layer FSS 100 configured to achieve a higher-order filter response is shown in FIG. 1. The phrase “higher-order”, as used herein, refers to an order greater than a first-order. As known in the art, in order to achieve a higher-order filter response, a plurality of first-order FSSs are cascaded by stacking respective FSSs to have a quarter wavelength spacing between each other.
FSS 100 is a third-order band-pass FSS and includes three (3) first-order FSSs 1021, . . . , 1023 separated by two (2) dielectric layers 1041, 1042. Each of the first-order FSSs 1021, . . . , 1023 can comprise an array of dipole or slot antennas that act as resonators around an operating frequency (e.g., 10 GHz) of the multi-layer FSS. Each of the dielectric layers 1041, 1042 act as an impedance inverter. The first-order FSSs 1021, . . . , 1023 are cascaded so as to have a certain distance d between each other. The distance d is a physical distance defined by the physical thickness of the respective dielectric layer 1041, 1042. The physical distance d typically has a value which corresponds to an electrical thickness of one-fourth of a wavelength (λ/4). For a frequency of ten gigahertz (10 GHz), one millimeter (1 mm) corresponds to one-thirtieth of a wavelength (λ/30). The third-order band-pass FSS 100 has an overall physical thickness t100. The physical thickness t100 is defined by the collective physical thickness of the two (2) dielectric layers 1041, 1042 since the FSS layers have negligible physical thicknesses in relation to the dielectric layers. The physical thickness t100 typically has a value that corresponds to an electrical thickness of one-half of a wavelength (λ/2). Thus, the physical thickness t100 of a multi-layer FSS increases linearly as the order of the FSS increases.
Notably, conventional FSSs (such as the FSS 100 of FIG. 1) suffer from certain known deficiencies. For example, the significant physical thickness t100 of the conventional FSS 100 results in an undesirable sensitivity of its response to the angle of incidence of the radiation. Also, the physical thickness t100 of conventional multi-layer FSS 100 limits its applications, including applications where conformal FSSs are required. Therefore, there is a need for an improved higher-order FSS design.